Of the two most commonly used methods for recording data, such as in a computer system, namely magnetic methods and optical methods, each has advantages. Magnetic systems are well understood and can record in an erasable mode. Optical systems, while often not erasable, can record data at a higher density than most magnetic systems. Magneto-optic systems are capable of combining features of both systems to provide a system which has a high data density and is also erasable.
Several magneto-optic recording schemes have been proposed. The scheme of most interest with respect to the present invention depends on the existence of a recording medium displaying two effects. The first is the Kerr magneto-optic effect. A material which displays this effect produces a change in the polarization of light which reflects from a magnetized area of the material. For example, polarized light which falls on a magnetized area will undergo rotation of the polarization plane upon reflectance from an area magnetized in a given polarity. A rotation in the opposite direction is produced by an oppositely-magnetized area.
The second effect is the effect under which media have a high coercive force at low temperatures and a low coercive force at high temperatures; i.e., under the second effect, the media are more easily magnetized at high temperatures than at low temperatures.
In a magneto-optic system, data can be recorded on the medium by using a heat source to heat an area of the medium above the temperature at which the medium is magnetizable and exposing the heated medium to a low global magnetic field. Upon cooling, the area which was heated and exposed to the magnetic field will be magnetized. Initially, the medium is normally bulk polarized in one direction. Thereafter, it is possible to detect whether a given area of the medium is or is not magnetized in a desired magnetic polarity by reflecting polarized light from the area and detecting rotation of the light. In this way, areas of a medium can be designated as binary digits or bits and the binary value of a bit can be assigned according to the presence or absence of a predetermined direction of rotation of polarized light, which will correspond to an upward or downward magnetization of the area of the medium. By reversing the polarity of the bias field and heating a written area, the area will return to the original bulk-magnetized direction. In this way, one can erase old information. Magneto-optic systems generally of this type are described in Mark H. Kryder, "Magneto-Optic Recording Technology", J. Appl. Phys., vol. 57, No. 1, pp. 3913-3918 and Nobutake Imamura, "Research Applies Magnetic Thin Films and The Magneto-optical Effect in Storage Devices", Journal of Electrical Engineering, March 1983, pp. 100-103.
A system of this type requires at least four parts: a medium displaying both the Kerr magneto-optic effect and the Curie effect; a heat source for heating an area of the medium to the magnetization temperature; a magnetic field source; and apparatus for detecting whether a given area of the medium has been magnetized with a given polarity. Each of these four components can be provided in a number of forms and configurations. Several factors are important in the selection of these components and moreover these factors are interrelated. The interrelationships are not always in a fortuitous sense, such that selection of a device with a given favorable factor may also necessitate accepting another unfavorable factor. Furthermore, the relationships between factors are not necessarily linear relationships so that changing a factor in a known amount produces changes in other factors which are not necessarily predictable. As a result of these relationships, it is not possible to select the four components based on a theoretical knowledge of the characteristics of the components.
Among the factors which enter into providing the four components are magnetic characteristics of the magnetic field source, characteristics of the media, and characteristics of the apparatus and method for changing polarity of the magnetic field source.
Particularly preferred in connection with the present invention is a magnetic field source which is a permanent magnet and a heat source which is a laser. A magneto-optical disk exerciser using a permanent magnet is briefly described in K. Ohta, et al. "Magneto-Optical Disk with Reflecting Layers", Proceedings, SPIE, volume 382, pp. 252-259. Use of a fine permanent magnet wire to magnetically write on an MnAlGe film is disclosed in R.C. Sherwood, et al. "MnAlGe Films for Magneto-Optic Applications", J. Appl. Physics, V. 42, No. 4, pp. 1704-1705. If a permanent magnet is used as a magnetic field source, the material selected for the permanent magnet will have a number of characteristics which affect other aspects of the system. The permanent magnet will produce a magnetic field having a particular spatial configuration. Of special interest will be the magnitude of the magnetic field strength at positions spaced from the surface of the permanent magnet. The configuration of the magnetic field of the magnet determines how close the magnet must be to the disk, places constraints on what type of disk material can be used, and how far the magnet must extend longitudinally past the recording medium in order to provide the necessary field strength at the edges of the medium. The field strength at the medium will be affected by disk wobble. Because field strength diminishes as the square of the distance variation can be decreased by using a stronger magnet at a greater distance. However, stronger magnets are typically larger and heavier and thus more difficult to turn.
The permanent magnet also has a mass density which is important in connection with moving the permanent magnet to produce a desired polarity of the magnetic field at the disk. When the permanent magnet is to be rotated around its longitudinal axis in order to write or erase a bit, the mass density of the magnet affects how much energy is consumed in rotating the magnet and how quickly the orientation of the magnet can be changed. The shape of the magnet, and in particular the cross-sectional shape of an elongated magnet will affect the strength and shape of the magnetic field, which in turn has the effects described above, and also has an effect on how close the magnet can be positioned to the medium and still provide for the clearance necessary when the magnet is moved or rotated. The cross-sectional shape also affects the moment of inertia about the longitudinal axis of the magnet and thus affects how much energy is consumed in rotating or moving the magnet.
The type of material selected for the medium affects the temperature and field strength which is needed in order to write or erase a bit. This factor in turn affects what type of magnetic material can be used for the permanent magnet, how close the permanent magnet must be positioned to the medium, which in turn can constrain the cross-sectional shape of the magnet needed to assure proper clearance with respect to the disk as the magnet is rotated. The selection of magnetic medium material also affects the price of the disks and may affect the type of substrate upon which the medium can be provided. The design of the apparatus for moving or rotating the permanent magnet also interacts with other aspects of the total design. In a system in which the permanent magnet is rotated by one or more electromagnets each producing a field that interacts with the permanent magnet field, the size and shape of the permanent magnet affects the shape of the electromagnet actuator coils because the electromagnet actuator coils must clear the permanent magnet as it rotates. The field strength of the permanent magnet at the location of interaction with the field produced by the actuator coils must be sufficiently strong to produce the desired rotation of the permanent magnet and thus the shape of the permanent magnet field places constraints on the types and particular shapes of actuator coils which can be used which, in turn, affects the amount of energy which is consumed in moving or rotating the permanent magnet.
Relatedly, the manner in which the actuator is used to rotate the magnet affects a number of aspects of the system. The amount of time allocated for rotationally accelerating the permanent magnet and the method and amount of time used to decelerate the magnet and to hold it in the preferred position during a read or write will affect the amount of energy which is consumed both instantaneously, and averaged over a single rotation of the magnet and will also affect how quickly the magnet can be changed from one polarity to another. The method of accelerating and decelerating will affect the type of circuitry which is needed to control the actuator.
Although the above list of factors is not necessarily complete, it suffices to indicate that the large number of factors, and their non-linear interrelatedness make selection of a magneto-optic system a complex and difficult task which becomes even more so if the intent is to provide a magneto-optic system which is commercially usable and can be produced economically.